1. Field of the Invention
The present invention concerns a method for achieving a thin film of solid material, this material possibly being a dielectric, a conductor or a semi-insulator. It may be crystalline or noncrystalline. It may consist of an amorphic or polycrystalline semiconductor whose crystallographic planes are oriented in any direction whatsoever. This material may have ferroelectric, piezoelectric, magnetic, electro-optic properties, etc.
A particularly interesting application of the method according to the invention concerns the achievement of ferroelectric capacitor memories.
2. Description of the Background
Various methods are known to achieve thin films of solid material. These methods depend on the nature of the material and the thickness of the film desired. For instance, it is possible to deposit thin films of a solid material on the surface of a piece by projection, spraying, electroplating, etc. A thin film may also be obtained by thinning a plate of the material desired by mechanical-chemical or chemical abrasion, the thin film obtained then being bonded or fixed on a piece serving as a support.
In general, a thin film is fixed on the surface of a piece so as to modify the properties of the piece superficially.
In the semiconductor field, it is also sometimes necessary to achieve thin semiconducting films, for example to manufacture so-called "silicon-on-insulator" substrates. Various techniques to achieve thin semiconducting films have been developed. One of the most recent techniques is based on the fact that the implantation of rare gas or hydrogen ions in a semiconducting material induces the formation of embrittled areas at a depth neighbouring the average ion penetration depth. Document FR-A-2 681 472 discloses a method which makes use of this property to obtain a thin film of semiconducting material. According to this method, a plate of the semiconducting material desired comprising a flat face is subjected to the following steps:
a first implantation step wherein the flat face of the plate is bombarded with ions, thus creating, within the plate body and at a depth neighbouring the ion penetration depth, a "gaseous microblister" layer separating the plate into a lower region making up the mass of the substrate and an upper region making up the thin film, the ions being chosen among rare gas or hydrogen gas ions; PA1 a second step wherein the flat face of the plate is placed in close contact with a support made up of at least a layer of rigid material. This close contact being achieved, for example, using an adhesive substance or through the effect of a prior preparation of the surfaces and possibly a heat and/or electrostatic treatment to favour the atomic bonds between the support and the plate; PA1 a third heat treatment step wherein the plate-support assembly is heated to a temperature higher than the temperature at which the implantation was performed and sufficient to create a separation between the thin film and the mass of the substrate. This temperature is higher than or equal to approximately 400.degree. C. for silicon. PA1 an ion implantation step during which one face of the substrate is bombarded with ions chosen among rare gas and hydrogen gas ions so as to create, in the body of the substrate at a depth neighbouring the average ion penetration depth, a layer of microcavities separating the substrate into two regions, PA1 a heat treatment step intended to heat the layer of microcavities to a temperature sufficient to bring about a separation between the two regions of the substrate, either naturally or by means of an applied stress.
This implantation is suited to create a layer of gaseous microblisters which will result in a rupture area at the end of the heat treatment. The layer of microblisters thus created in the plate body, at a depth neighbouring the average ion penetration depth, delimits two regions in the plate body separated by this layer: a region intended to make up the thin film and a region forming the rest of the substrate. During the third step, the heat treatment is performed at a temperature sufficient to create the rupture area and the separation between the two regions through a crystalline rearrangement effect in the semiconducting material, for example through a microcavity growth effect and/or a microblister pressure effect.
Depending on the implantation conditions, following the implantation of a gas such as hydrogen for example, cavities or microblisters may or may not be observed with a transmission electron microscope. In the case of silicon, microcavities of sizes ranging from a few nm to a few hundred nm may be present. As a result, these cavities are only observable in the heat treatment step, particularly when the implantation temperature is low, and nucleation is therefore performed during this step so as to obtain the rupture between the thin film and the rest of the substrate at the end of the heat treatment.
Until now, it was believed that the method disclosed in document FR-A-2 681 472 could only be applied to achieve a thin film using a substrate of semiconducting material. This document provides the following explanation to the various phenomena known from experience. First of all, the first ion implantation step is carried out by exposing a flat face of a plate of semiconducting material to a beam of ions, the plane of this flat face being either substantially parallel to a main crystallographic plane in the case where the semiconducting material is perfectly monocrystalline, or slightly inclined with respect to a main crystallographic plane with the same index for all grains in the case where the material is polycrystalline. This leads to the creation, in the plate body at a depth neighbouring the average ion penetration depth, of a layer of "gaseous microblisters" corresponding to embrittlement areas and delimiting, in the plate body, two regions separated by this layer: a region intended to make up the thin film and a region forming the rest of the substrate. The term "gaseous microblisters" refers to any cavity or microcavity generated by the implantation of hydrogen gas or rare gas ions in the material. The cavities may have either a very flat shape, i.e. with a small height, for example in the order of a few atomic gaps, or a substantially spherical shape or any other shape different from the two previous shapes. These cavities may or may not contain a free gaseous phase and/or gas atoms derived from implanted ions fixed to atoms of the material forming the walls of the cavities. These cavities are generally referred to as "microblisters", "platelets or even " "bubbles". During the third step, the heat treatment is performed at a temperature sufficient (for the duration of the treatment applied) to create the separation between the two regions. The time-temperature pair of the heat treatment depends of the dose of implanted ions.
The method described in document FR-A-2 681 472 concerns the achievement of a thin film using a substrate of semiconducting material having a crystalline structure. The development of the various steps of the method has been explained as resulting from the interaction between the implanted ions and the crystalline mesh of the semiconducting material.
However, the inventors of the present invention were surprised to discover that this method could be applied to all types of solid materials, crystalline or noncrystalline. This method may be applied to dielectric, conducting, semi-insulating materials, as well as to amorphic semiconducting materials and even polycrystalline semiconductors whose grains do not have main crystallographic planes substantially parallel to the flat face of the plate. The latter, along with amorphic semiconductors, will be referred to as unorganized semiconductors hereinafter. Furthermore, this method does not significantly modify the properties of the material it is applied to.
The inventors of the present invention were surprised to discover that the implantation of hydrogen gas or rare gas ions may also bring about the formation of microcavities in solid materials other than a crystalline semiconducting material, and that a subsequent heat treatment may bring about the separation, at the microcavities, of the mass of the material into two parts. Indeed, the heat treatment induces, regardless of the type of solid material, the microcavities to coalesce, which leads to an embrittlement of the structure at the layer of microcavities. This embrittlement enables the separation of the material under the effect of internal stresses and/or pressure within the microcavities, this separation being natural or assisted by external stresses.
The term layer of microcavities refers to an area containing microcavities possibly located at different depths and adjacent or not adjacent to one another.